1. Field of the Invention
The present invention relates to a method of fabricating a superconducting quantum interference device (SQUID) constructed from short weak links with ultrafine metallic wires. The principle of the fabrication of the ultrafine metallic wires is based on the field evaporation phemomenon of atoms of superconducting materials. An elastic field is applied to the submicron area edge-sandwich.
2. Description of the Related Art
A superconducting quantum interference device is capable of detecting a magnetic field with high sensitivity and has been used for the measurement of magnetic susceptibility, for the detection of nuclear magnetic resonance signals and for biomagnetic measurements. To fabricate thin film SQUIDS, it is necessary to use superconducting thin films of very high quality having a critical temperature which in principle is equal to that of a bulk superconductor, even if its thickness is in the range of several nanometers. When fabricating, for example, a weak link type Josephson element with the structure of a metallic bridge, nanometer patterning techniques are required, for forming a weak link of submicron dimensions (i.e., length, width and thickness). The length of a weak link should be up to three to five times the superconducting coherence length (e.g., 10 nm for Nb at 4.2K, 4 to 5 nm for NbN), which is a very severe condition to fulfil. On the other hand, when a tunnel junction is used as a SQUID element, the precise control of the thickness and uniformity of the tunnel barrier become crucially important.
Because, as follows from the above, the SQUID fabrication requires very advanced processing techniques, the widespread use of SQUIDS has been somewhat delayed. Only recently, the fluxmeters incorporating a tunnel type DC-SQUID have been applied for practical measurements of magnetic fields produced by the human brain.
FIG. 3 shows the general construction of a DC-SQUID. The DC-SQUID consists of two Josephson elements 6 and 7 connected in single loop 5. The DC-SQUID detects external magnetic fields passing through the loop due to the effect thereof on the interference of superconducting electrons moving across the Josephson elements 6 and 7. As a result, the current-voltage characteristic of the DC-SQUID varies periodically with external magnetic flux at the period .PHI.0=2.07.times.10.sup.-15 Wb. Such a current-voltage characteristic enables the measurement of magnetic flux with high sensitivity.
FIG. 5 shows an example of a DC-SQUID. There are shown the counter electrode 8 forming the superconducting loop, the base electrode 9 formed at the open end of the loop, junction windows 10 in which weak links 11 are formed between the counter electrode 8 and the base electrode 9, and an input coil for coupling signal magnetic flux to SQUID loop. The signal magnetic flux may be detected through the variation of the voltage between the counter and base electrodes 8 and 9.
A Josephson element that exhibits the Josephson effect is formed by weakly linking two superconductors. Josephson elements are classified into those of a tunnel junction type, a weak link type (bridge type), a proximity effect type and a point-contact type. Various methods of fabricating Josephson elements have been proposed. FIGS. 4(a) to 4(d) show an example of the fabrication steps of an edge tunnel type Josephson element as disclosed in Appl. Phys. Lett. 55 81 (1989). To get a small area junction, for a high sensitivity DC-SQUID, the tunnel type Josephson element is prepared on the oblique edge of the superconductive NbN film and consists of NbN/MgO/NbN trilayer. First, as shown in FIG. 4(a), a niobium nitride layer 14 and an aluminum layer 15 as connection electrodes are formed by vacuum evaporation, an alumina film 16 is formed to protect the niobium nitride layer 14 as shown in FIG. 4(b), the niobium nitride film is etched to form an edge as shown in FIG. 4(c), and then, a MgO barrier thin film and a niobium nitride counterelectrode 17 are formed as shown in FIG. 4(d). The niobium nitride layer 14 serves as a base electrode. The niobium nitride layer 14, the counterelectrode 17 and the MgO barrier thin film formed between the niobium nitride layer 14 and the counterelectrode 17 constitute an edge tunnel junction 18 using edge of NbN film.
A DC-SQUID is obtained by forming two Josephson elements in a superconducting loop. As is generally known, the minimum detectable magnetic flux, which can be detected by the DC-SQUID, decreases with a reduction of the loop inductance L.sub.s and the capacitances of the Josephson junction elements. However, the loop inductance L.sub.s cannot be arbitrarily reduced because the efficiency of magnetic flux transfer between the pick-up coil and the DC-SQUID loop must be held at a proper level. Accordingly, the capacitances C of the Josephson elements need to be reduced to reduce the minimum detectable magnetic flux.
The capacitance C of the edge tunnel type Josephson element is dependent on a ratio A/d, where A is the area of the counterelectrode and d is the thickness of the tunnel barrier. Therefore, the capacitance can be reduced by decreasing the area A and increasing the thickness of tunnel barrier d. However, the characteristics of the tunnel type Josephson element limit the thickness d to a value not greater than about 1-2 nm. Accordingly, the prior art DC-SQUID has a small-area Josephson junction provided with a barrier thin film having a thickness on the order of 1 nm.
It is not easy to form such thin film. So, it is very desirable to develop a fabrication technology which is much less dependent on the barrier properties.
Fabricating the Josephson element with a small area is effective for preventing magnetic flux trapping in the junction and is achieved by using the edge of a thin film as shown in the example of FIG. 4.
The thickness of the barrier film of the tunnel type Josephson element is on the order of about 1 nm. A change of about 0.5 nm in the thickness of the barrier film will cause one order of magnitude change in the critical current density. Advanced technology is required for controlling the barrier thickness accurately to 0.1 nm. The adjustment of the thickness of the barrier thin film is an important technical problem when fabricating a tunnel-type DC-SQUID and thus the fabrication of a DC-SQUID requires precision processing techniques in forming the superconducting and insulator (tunnel barrier) layers.
If the pair of Josephson elements formed in the loop of a DC-SQUID do not have equal characteristics, the sensitivity of the DC-SQUID decreases. Hence, the DC-SQUID must be fabricated with a pair of Josephson elements which have equal characteristics. However, as mentioned above, the characteristics of a Josephson element are very difficult to control. So far, a DC-SQUID with weak links has not reproduced the performance level of a SQUID. Furthermore, it is very difficult to form two Josephson elements having equal characteristics. Tunnel-type DC-SQUIDS have been also fabricated by only few advanced companies with control of the tunnel barrier to be accurate to 0.1 nm. In the prior art a DC-SQUID with a tunnel has been provided with an external shunt resistor to cancel the large capacitance within, however, requires a complicated process for forming the DC-SQUID.